Without limiting the scope of the invention, its background is described in connection with optical waveguides.
High index contrast silicon/silicon dioxide optical waveguides can radiate 60 to 100% of the light propagating in the waveguide from a grating etched into one surface of the waveguide in short distances—in about 10 to 20 microns (or about 20 to 40 grating periods). The period of these radiating gratings is typically at or near the second Bragg condition, meaning that the grating period is equal, or approximately equal to the wavelength of the radiated light propagating in the waveguide (or the free space wavelength λo divided by the effective index). At or near the second Bragg condition, there can be a significant second-order Bragg in-plane reflection. This often undesired reflection could be eliminated by tilting the radiated light (e.g., with an appropriate choice of the grating period) sufficiently off of the axis normal to the laser surface, or by the addition of one or more additional slits (grating grooves or ridges) appropriately spaced away from the coupler grating. The additional slit(s) serve as a partially reflecting mirror and by destructive interference cancel the in-plane reflection.
However, generating light on a silicon wafer is problematic. There are numerous other semiconductors, often alloys of elements in columns III and V (III-V compounds—which include GaAs and InP alloys) and columns II and VI (II-VI compounds) of the periodic table that commonly generate light and from which semiconductor light-emitting diodes (LEDs) and semiconductor lasers are fabricated. Being able to couple light out of a short section of a waveguide formed in a compound semiconductor optical waveguide has several advantages including: 1) optimum coupling to single- and multi-mode optical fibers; 2) optimum coupling to silicon photonic waveguides; and 3) economic gains by reducing the real estate used by the grating coupler.
One example is taught in U.S. Pat. No. 7,006,732, issued to Gunn, III, et al., entitled, “Polarization splitting grating couplers.” Briefly, this patent teaches a polarization splitting grating coupler (PSGC) that connects an optical signal from an optical element, such as a fiber, to an optoelectronic integrated circuit. The PSGC is said to separate a received optical signal into two orthogonal polarizations and to direct the two polarizations to separate waveguides on an integrated circuit.
Another example is taught in U.S. Pat. No. 7,068,887, also by Gunn, III, et al., entitled, “Polarization splitting grating couplers.” Again, a polarization splitting grating coupler (PSGC) is said to connect to an optical signal from an optical element, such as a fiber, to an optoelectronic integrated circuit, and is capable of separating a received optical signal into two orthogonal polarizations, and directs the two polarizations to separate waveguides on an integrated circuit. The two separated polarizations can then be processed, as needed for a particular application by the integrated circuit. The PSGC can also operate in the reverse direction.
Another example is taught in U.S. Pat. No. 6,760,359, also by present inventor (Evans), and is entitled, “Grating-outcoupled surface-emitting lasers with flared gain regions.” Briefly, a laser system is taught that includes a laser diode with an active region and reflectors at both ends. An outcoupling aperture is located between the reflectors to couple light out of the device through the surface. The gain region increases in width as it nears the outcoupling aperture.
High index contrast silicon/silicon dioxide optical waveguides can radiate 60 to 100% of the light propagating in the waveguide from a grating etched into one surface of the waveguide in short distances—in about 10 to 20 microns (or about 20 to 40 grating periods) at a wavelength of 1550 nm. The term “light” and “optical” refer to electromagnetic waves that extend to wavelengths shorter (ultraviolet) and longer (infrared) than light visible to the human eye. Presently semiconductor LEDs and lasers span the wavelength range from about 0.3 microns to many tens of microns.
Despite many advances in the field, a need remains to enhance coupling of electromagnetic radiation due to gratings.